Exhaust Gas Treatment Device and Method for Operating an Exhaust Gas Treatment Device

ABSTRACT

An exhaust gas treatment device for an internal combustion engine, with a first particle filter and a second particle filter arranged downstream of the first particle filter in the flow direction of the exhaust gas through the exhaust gas treatment device. The second particle filter includes a support device that can be flown through by the exhaust gas, on which a filter unit is disposed. A pore size of the filter unit is less than a pore size of the support device. A method for operating an exhaust gas treatment device for an internal combustion engine is also provided.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to an exhaust treatment device for an internalcombustion engine, with a first particle filter and a second particlefilter arranged downstream of the first particle filter in the flowdirection of the exhaust gas through the exhaust gas treatment device.The invention further relates to a method for operating an exhaust gastreatment device for an internal combustion engine.

PCT Publication No. WO 2008/043422 A1 describes an exhaust treatmentdevice for an internal combustion engine in which a second particlefilter is arranged downstream of a first particle filter. A volume ofthe second particle filter is preferably less than 30% of the firstparticle filter, so that the second particle filter has a lower particlereception capacity than the first particle. Additionally oralternatively, a lower porosity of the second particle filter comparedto the first particle filter can lead to a reduced particle receptioncapacity of the second particle filter. Due to the relatively lowparticle reception capacity of the second particle filter, an increaseof the loading of the second particle filter can be detected relativelyquickly. A malfunction of the first particle filter or a reduced degreeof separation of the particles at the first particle filter can thus berecognized so that measures can be made to restore the functioning ofthe first particle filter.

Exemplary embodiments of the present invention provide an exhaust gastreatment device or a method of the above-mentioned type, by means ofwhich an impairment of the functioning of the first particle filter canbe determined even quicker.

The exhaust gas treatment device for an internal combustion engineaccording to the invention has a first particle filter and a secondparticle filter arranged downstream of the first particle filter in theflow direction of the exhaust gas through the exhaust gas treatmentdevice. The second particle filter includes at least one support devicethat can be flown through by the exhaust gas, on which a filter unit isarranged, wherein a pore size of the filter unit is smaller than a poresize of the at least one support device.

The invention is based on the knowledge that, with a second particlefilter known from the state of the art, a reduction of its particlereception capacity has limitations by providing a particularly smallvolume thereof, which restricts its mechanical stability. Presently, theporous support device that can be flown through by the exhaust gasensures the sufficient mechanical stability of the second particlefilter. The second filter unit, which has a smaller pore size comparedto the support device, can thereby be comparatively delicate, and can bedesigned optimally to the particularly effective separation ofparticles.

For example, the filter unit is formed as filter membrane applied to thesupport device. A loading of the filter membrane then increases veryquickly due to a damage, such as a crack, of the first particle filter.A duration up to the determination of the impairment of thefunctionality of the first particle filter is then particularly low.This ensures a particularly high accuracy when determining a reductionof a separation degree of the first particle filter. Furthermore, bymeans of this formation of the second particle filter, its installationspace requirement, weight and cost are particularly low. Moreover, thesecond particle filter only slightly increases the counter pressure ofthe exhaust gas treatment device.

In an advantageous arrangement of the invention, the pore size of thefilter unit is at least half, in particular half to one-tenth of thepore size of at least one support device. The pore size of the filterunit can also, for example, be a fifth of the pore size of the at leastone support device. This is particularly effective to ensure that theretention of particles in the exhaust gas flow takes place through thefilter. Thus, an increase of the loading of the second particle filtercan be sensed detected very quickly and accurately.

It has further been shown to be advantageous if the filter unit forretaining particles is formed by surface filtration. The filter unit canthus be passed through by the exhaust gas with a very low pressure loss.However, soot particles cannot penetrate into the filter unit, but areretained on the surface of the filter unit.

The filter unit can have micro-filtration properties so that sootparticles with a diameter of less than 10 mμ, particularly sootparticles with a diameter of 10 mμ to 0.1 mμ, can be retained very wellby the filter unit.

In a further advantageous arrangement of the invention, a thickness ofat least one support device is a multiple of the thickness of the filterunit. Because the filter unit is designed so as to be particularly thincompared to the support device, the filter unit has a good flow-throughcapacity for the exhaust gas. Alternatively, but preferably in addition,a porosity of at least one support device is a multiple of the porosityof the filter unit. The filter unit thus having few and comparativelyfine pores compared to the support device, very quickly blocks particlespassing due to damage of the first particle filter, such as when thelatter has as a tear or a hole as a result of repeated temperaturechanges or due to a long operation period. A concomitant significantincrease of the pressure drop across the second particle filter can alsobe sensed with less sensitive and therefore very robust pressure sensingmeans in a very simple and clear manner.

In another advantageous embodiment of the invention, a smallestextension dimension of the at least one support device is a multiple,especially at least five times, of a thickness of at least one supportdevice transversely to the flow direction of the exhaust gas. Forexample, with a support device formed as a porous round disk, thediameter is thus a multiple, particularly the five fold or the ten foldof the thickness of the support device. By means of this flat design ofthe support device, its flow through resistance is particularly low.

It has further been shown to be advantageous if the at least one supportdevice and the filter unit are formed at least partially from anidentical material. Tensions between the support device and the filterunit due to a different expansion behavior with a temperature increasecan thereby be avoided. This coincides with a very high durability ofthe second particle filter.

The material forming the support device and the filter unit formingmaterial can be a metallic or ceramic material. For example, the supportdevice can be formed as a metallic wire mesh, as a single layer ormultilayer metal sieve or as a metal foam component. The filter unitformed, for example, as a filter membrane filter unit can be a densemetal fiber fleece. Such a fleece of metal fibers preferably has afelt-like structure. Fibers of the fleece can be connected to each otherduring the manufacture of the filter unit and/or of the second particlefilter, for example by sintering. In particular, a connecting of thesupport device and filter unit can take place by sintering. If thefilter unit is manufactured separately, it can be connected to thesupport device by soldering or welding. Instead of a separate productionof the filter unit and a subsequent connection to the support device,the support device and the filter unit can alternatively be produced inthe same production process.

If the support device is formed from a ceramic material, such as highlyporous ceramic component, the filter unit is preferably also formed froma porous ceramic material.

Metallic and ceramic materials used for the support device and thefilter unit are advantageously resistant to high temperatures, so thatan arrangement in great proximity to the first particle can be provided.These high temperature-resistant materials withstand a temperature loadof 900° C., particularly from 1200° C. to 1300° C. without damage. Thisis particularly beneficial if high temperatures occur in the exhaust gastreatment device during the regeneration of the particle filter, thusduring the combustion of soot deposited on the particle filter.

The at least one support device can be arranged downstream and/orupstream of the filter unit. If the support device is arranged upstreamand downstream of the filter unit, a material-fit connection of thefilter unit to at least one of the support units can be omitted, as thefilter unit can be fixed in its position by the support devices arrangedon both sides.

If the at least one support device is arranged upstream of the filterunit, the support device can take on a separation function in additionto its support function. The second particle filter then also hasproperties of a depth filter, as a separation of particles takes placein channels and/or pores of the support device and on the surface of thefilter unit.

If the filter unit is arranged between two support devices, the twosupport devices can differ with regard to their structure and/or theirpore size with regard to the materials respectively used for them.

The second particle filter can at least substantially be arrangedvertically or inclined to the flow direction of the exhaust gas in theexhaust gas treatment device. By an arrangement inclined to the flowdirection off exhaust gas in the exhaust gas treatment device, anenlargement of the surface of the second particle filter effective forseparating particles can be achieved. With an installation vertical tothe flow direction, a particularly low space requirement is howevergiven.

In order to fix the second particle filter to an inner wall of theexhaust gas treatment device, the support device can be connected to theinner wall connected by welding or soldering. Additionally oralternatively, a fiber mat or the like can be provided between the innerwall of the exhaust gas treatment device and the least one supportdevice, wherein the support device is fixed in position in the exhaustgas treatment device by a clamping effect of the fiber mat. Thisclamping effect is enhanced in particular by extending the fiber mat atan increased temperature of the exhaust gas treatment device.

For increasing an effective filter surface of second particle filter,the second particle filter can have at least one elevation. Theelevation is hereby preferably aligned against the flow direction of theexhaust gas to prevent an accumulation of particles in a narrow regionof the second particle filter. In particular, a plurality of concentricelevations can be provided. Additionally or alternatively, the secondparticle filter can have a plurality of concentric elevations, so thatthe second particle filter has a corrugated profile in its longitudinalsection.

It has further been shown to be advantageous, if the second particlefilter is arranged in the exhaust gas to be regenerated by a heattransfer, wherein the heat transfer can be effected by a thermalregeneration of the first particle filter. This ensures that acombustion of soot, which was separated from the exhaust gas by means ofthe first particle filter, leads to a combustion of soot at the secondparticle filter. A separate regeneration of the second particle filtercan thus be omitted.

In order to effect a particularly efficient heat transfer from the firstparticle filter to the second particle filter, the second particlefilter can be arranged within a housing surrounding the first particlefilter. A distance of a few centimeters between the first particlefilter and the second particle filter can be provided hereby.Alternatively, the first particle filter and the second particle filtercan contact each other, at least in some regions.

Alternatively, the second particle filter can be accommodated in aseparate housing, which has an enlarged cross section compared to aregion of the exhaust gas tract upstream of this separate housing. Byarranging the second particle filter in the housing surrounding thefirst particle filter or in the separate housing, the second particlefilter can therefore have a particularly large cross section that can beflown through, and, consequently, have a particularly low flow-throughresistance.

In that the second particle filter is also regenerated when regeneratingthe first particle filter, a continuous increase of the loading of thesecond particle filter as a result of operating the intact firstparticle filter can be avoided. As long as the first particle filter isintact and has a sufficient particle reception capacity for maintaininglegal requirements, no significant increase of the exhaust gas counterpressure will be observed at the second particle filter.

According to a further advantageous arrangement of the invention, theexhaust gas treatment device has pressure sensing means and anevaluation device, wherein a difference of the pressure between alocation upstream of the second particle filter and a locationdownstream of the second particle filter with regard to a state of thefirst particle filter can be evaluated by means of the evaluationdevice. The difference can hereby be determined by means of adifferential pressure sensor or by means of two absolute pressuresensors. If the value of the difference exceeds a threshold, it can beconcluded that the first particle filter is damaged. In contrast, whenfalling below the threshold, it can be concluded that the first particlefilter is not in a critical state.

Such an evaluation of the difference with regard to the state of thefirst particle filter is particularly safe and fast. An evaluation of atemperature increase on the second particle during the thermalregeneration thereof that can be provided additionally or alternativelywould, however, only enable a detection of a damage of the firstparticle filter during the regeneration. As the thermal regeneration iscarried out in differently sized time intervals, damage of the firstparticle filter can then not always be detected immediately. Incontrast, when sensing the pressure drop across the second particlefilter, an already existing measuring device for sensing the pressuredrop across the first particle filter, particularly a measuring line,can also be used.

It has hereby been shown to be advantageous if a reference value of thedifference characterizing the state of the first particle filter isstored in a memory. The memory can be arranged in the evaluation unit.The reference value of the difference preferably characterizes a newstate of the first and the second particle filter with the initialoperation of the exhaust gas treatment device. If a value of thedifference that is larger than a threshold compared to the referencevalue for the new state of the first particle filter is detected, afaulty state of the first particle filter can be concluded by means ofthe evaluation unit. Such a faulty state of the first particle filtercan be communicated via a communication device, in the form of anoptical and/or acoustic alarm signal or a display or the like.

The reference value characterizing the state of the first particlefilter is preferably variable in dependence on an aging of the firstparticle filter. The reference value can be stored in the memory in theform of a characteristic line, which considers an operation duration ofthe exhaust gas treatment device and/or an exhaust gas flow rate throughthe exhaust gas treatment device and/or a fuel flow rate of the internalcombustion engine comprising the exhaust gas treatment device and/orwith an exhaust gas treatment device arranged in a vehicle, a distancetraveled of the vehicle. Additionally or alternatively, temperatureprofiles of the gas treatment device can be consulted for thecharacterization of the aging of the first particle filter.

In order to prevent a short-term exceeding of the threshold leading torecognition of damage of the first particle filter, the value of thedifference formed from the measuring values can be damped and/or,particularly with the use of a low pass filter, processed in theevaluation unit in a filtered manner or passed to the evaluation devicein a damped or filtered manner.

The pressure drop across the second particle filter can particularly beevaluated continuously. A standardisation preferably takes place hereby,in order to obtain a value independent of the interfused exhaust gasvolume flow.

In a further advantageous arrangement of the invention, the exhaust gastreatment device has pressure sensing means for sensing the pressureupstream and downstream of the first particle filter. The loading stateof the first particle filter can thereby be determined in dependence ona pressure drop at the first particle filter. A differential pressuresensor can be used here. An absolute pressure sensor can respectively beprovided upstream and downstream of the first particle filter. In analternative embodiment, only one absolute pressure sensor is present ata location upstream of the first particle filter, and the pressuredownstream of the first particle filter is calculated using a model.

If the pressure drop at the first particle is also sensed, this size canbe used for the plausibility of the state of the first particle filterdetermined via the measuring of the pressure drop at the second particlefilter. Furthermore, the measuring location of the pressure sensingmeans arranged downstream of the first particle filter can be used fordetermining the pressure drop at the first particle filter and fordetermining the pressure drop at the second particle filter.

In a supplementary or alternative embodiment of the invention, where theexhaust gas treatment device has pressure sensing means and anevaluating device, the pressure sensing means can be used to determine aratio of a difference in pressure between a location upstream of thesecond particle filter and a location downstream of the second particlefilter to a difference of the pressure between a location upstream ofthe first particle filter and a location downstream of the firstparticle filter. A ratio determined by means of the pressure sensingmeans can be compared to a ratio deposited in a memory by means of theevaluating unit and can be evaluated with regard to a state of the firstparticle filter.

The ratio stored in the memory is usually the highest in the new stateof the gas treatment device and decreases with increasing operating timeof the exhaust gas treatment device. Damage of the first particle filtercan be determined by means of evaluation device, if the ratio determinedby means of the pressure sensing means increases again during operation.

In a further advantageous embodiment of the invention, the evaluation isdesigned to evaluate the ratio determined by means of the pressuresensing means in dependence on a thermal regeneration at least of thefirst particle filter. This is based on the knowledge that the ratiodeposited in the memory characterizing the new state of the exhaust gastreatment device can, at best, be reached if the first particle filterand the second particle filter were just regenerated. It can thus beprovided to always evaluate the ratio determined by means of thepressure sensing means if a thermal regeneration of the first particlefilter has just taken place. An average value of the ratio within apredetermined time window can be determined hereby for example.

It has finally been shown to be advantageous if the exhaust gastreatment device comprises an exhaust gas return line branching offdownstream of the second particle filter, particularly comprising athrottle device. Accordingly, the second particle filter serves for theretention of particles coming from the first particle filter, which arepresent there, possibly due to a damage or due to the production of thefirst particle filter.

In particular, when the exhaust gas recirculation line opens upstream ofa compressor of an exhaust gas turbocharger in an intake tract of theinternal combustion engine, the second particle filter serves as aprotection of the compressor from damage due to such particles. Inaddition, the second particle filter arranged upstream of the branch ofthe exhaust gas return line does not contribute to an increase of thecounter pressure of the exhaust gas return line, so that has the exhaustgas return line has an undiminished effective scavenging gradient.Arranging the second particle filter upstream of the branch of theexhaust gas return line further avoids an increasing loading of thesecond particle filter with the operation of the exhaust gas return lineleading to an impairment of the functioning of the exhaust gas returnwith an intact first particle.

A further aspect of the invention provides a method for operating anexhaust treatment device for an internal combustion engine in whichexhaust gas is filtered by means of a first particle filter and a secondparticle filter arranged downstream of the first particle filter in theflow direction of the exhaust gas through the exhaust gas treatmentdevice. The exhaust gas filtered by means of the second particle filterflows through at least one support device and a filter unit arranged atthe at least one support device, with a pore size of the filter unitbeing smaller than a pore size of the at least one support device. Thepreferred embodiments and advantages described for the exhaust gastreatment device according to the invention also apply to the method foroperating an exhaust treatment device. according to the invention.

The characteristics and characteristic combinations mentioned above inthe description and the characteristics and characteristic combinationsmentioned in the following in the figure description and or shown alonein the figures cannot only be used in the respectively givencombination, but also in other combination or on their own, withoutleaving the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Further advantages, characteristics and details of the invention resultfrom the claims, the following description of preferred embodiments andfrom the drawings. It shows thereby:

FIG. 1 a schematic representation of an internal combustion engine of avehicle with an air intake tract and exhaust gas tract, which comprisesan exhaust gas treatment device with a first particle filter and asecond particle filter arranged downstream of the first particle filter;

FIG. 2 sectional views of three alternative designs of the secondparticle filter;

FIG. 3 in a sectional view, a first type of the arrangement of thesecond particle filter in the exhaust gas tract;

FIG. 4 in a sectional view, an alternative type of the arrangement ofthe second particle filter in the exhaust gas tract, wherein the secondparticle filter has an alternative design;

FIG. 5 in a sectional view, a further design of the second particlefilter arranged in the exhaust tract;

FIG. 6 a sectional view and a top view of a further design of the secondparticle filter in the flow direction of the exhaust gas;

FIG. 7 a sectional view and a top view of a design of the secondparticle filter alternative to the design in FIG. 6 in the flowdirection of the exhaust gas;

FIG. 8 a section of the exhaust gas tract according to FIG. 1 with afirst example of an arrangement of pressure sensing means;

FIG. 9 a section of the exhaust gas tract according to FIG. 1 with asecond example of an arrangement of pressure sensing means;

FIG. 10 a section of the exhaust gas tract according to FIG. 1 with athird example of an arrangement of pressure sensing means;

FIG. 11 a section of the exhaust gas tract according to FIG. 1 with afourth example of an arrangement of pressure sensing means;

FIG. 12 a section of the exhaust gas tract according to FIG. 1 with afifth example of an arrangement of pressure sensing means;

FIG. 13 a section of the exhaust gas tract according to FIG. 1 with asixth example of an arrangement of pressure sensing means;

FIG. 14 a section of the exhaust gas tract according to FIG. 1 with aseventh example of an arrangement of pressure sensing means; and

FIG. 15 a section of the exhaust gas tract according to FIG. 1 with aneighth example of an arrangement of pressure sensing means.

DETAILED DESCRIPTION

FIG. 1 schematically shows an internal combustion engine 1 of a vehiclewith an air intake tract 11 and an exhaust gas tract 3. Exhaust gasleaving the internal combustion engine 1 flows over a turbine 2 of anexhaust gas turbocharger, which drives a compressor 12. In the exhausttract 3 is arranged an exhaust gas treatment device 13, where presentlya catalyst 4, a first particle filter 5 and a second particle 6 arearranged in a housing 14. In the flow direction of the exhaust gas,which is specified in the exhaust gas tract 3 by flow arrows 15, thefirst particle filter 5 is hereby arranged downstream of the catalyst 4and the second particle filter 6 downstream of the first particlefilter. The housing 14 surrounding the particle filters 5, 6 and thecatalyst 4 has a widened cross section compared to a region of theexhaust gas tract 3 upstream and downstream of this housing 14.

Downstream of the housing 14 and the exhaust gas tract 3, a branch of anexhaust gas return line 8 is provided. An exhaust gas cooler 9 isarranged in the exhaust gas return line 8. The exhaust gas return line 8serves for a low-pressure exhaust gas return, wherein the returnedexhaust gas upstream of the compressor 12 is guided to the air intaketract 11 of the internal combustion engine 1. Before the entry of theexhaust gas return line 8 into the air intake tract 11 a throttlingdevice 10 is arranged for throttling the exhaust gas flow through theexhaust gas return line 8. A further throttling device 7 is arrangeddownstream of the branch of the exhaust gas return line 8 in the exhaustgas tract 3.

The second particle filter 6 has, seen in the flow direction of theexhaust gas through the housing 14, a thickness which is lower by amultiple than the first particle filter 5. A particle reception capacityof the second particle filter 6 is therefore considerably smaller than aparticle reception capacity of the first particle filter 5.

The second particle filter 6 presently has two different functions. Onthe one hand, the second particle filter 6 retains particles coming fromthe first particle filter 5 and/or the catalyst 4, such as productionresidues or particles dislodged from damage. These particles couldotherwise enter the exhaust gas return line 8 and thus possibly damagethe compressor 12 of the exhaust gas turbocharger.

In addition to this component protection, the second particle filter 6monitors the functionality of the first particle filter 5. The secondparticle filter 6, which has the particularly low particle receptioncapacity compared to the first particle filter, reacts extremelysensitively to an increase of its load, possibly as a result of a damageof the first particle filter 5. Such damage may be a crack and/or a holein the first particle filter 5. If the first particle filter 5 has suchdamage, and its functionality with regard to the effective retaining ofparticles is thus restricted, the loading of the second particle filter6 increases abruptly. The increase of the load of the second particlefilter 6 in the case of a damage of the first particle filter 5 takesplace particularly fast and reliably due to the design of the secondparticle filter 6. The increase of the load can be detected via apressure drop at the second particle filter 6 coinciding with the load.

Three different examples for specific designs of the second particlefilter 6 are given in enlarged, schematic representations in FIG. 2. Thesecond particle filter 6 accordingly comprises at least one supportdevice 16 that can be flown through by the exhaust gas, which serves asa support for a filter membrane 17.

The support device 16 can be formed from a metallic wire mesh, on whichthe filter membrane 17, possibly in the form of a dense metal fiberfleece, is fixed. The support device 16 can also be a, in particularsingle-or multi-layer, screen or be formed as metal foam component.Alternatively, a highly porous ceramic can be provided as the supportdevice 16. The filter membrane 17 is then also preferably formed as aceramic filter membrane 17. A pore size of the filter membrane 17 ispresently only a fraction of the pore size of the supporting device 16.For example, the pore size of the filter membrane 17 can be half, afifth or even only one-tenth of the pore size of the support device 16.

A retention of particles through the filter membrane 17 takes place bysurface filtration. Soot particles can thus not penetrate the filtermembrane 17. The filter membrane 17 further has a porosity smaller,particularly by a multiple, compared to the support device 16. Thethickness of the filter membrane 17 in the flow direction of the exhaustgas through the second particle filter 6 is also several times smallerthan a thickness of the support device 16.

The thin filter membrane 17 supported by the support device 16 in thehousing 14 thereby enables a particularly quick and exact detection of asoot entry into the second particle filter 6. Thus, an operationduration up to the recognition of a damage of the first particle filter5 is particularly low. The filter membrane 17 nevertheless has aparticularly good flow through capacity for exhaust gas due to its smallthickness and thus increases the overall counter pressure of the exhausttreatment device 13 at most minimally, if the filter membrane 17 is notloaded largely due to damage of the first particle filter 5.

According to a first design of the second particle filter 6 shown inFIG. 2, the support device 16 is arranged downstream of the filtermembrane 17. If alternatively, as also shown in FIG. 2, the supportdevice 16 is arranged upstream of the filter membrane 17, the supportdevice 16 also contributes to the separating of particles from theexhaust gas flow, wherein a retention of particles by means of depthfiltration can then take place in the region of the support device 16.According to a third design of the second particle filter 6 shown inFIG. 2, the filter membrane 17 is arranged as an intermediate layerbetween two support devices 16.

FIG. 3 shows the second particle filter 6 in a sectional view, as it canbe arranged in an exemplary manner in the housing 14. Here, the secondparticle filter 6 is arranged vertical to the flow direction of theexhaust gas in the housing 14. Presently, the housing 14 is formed roundin its cross section in the region of the second particle filter. Thesecond particle filter 6 in the design according to FIG. 3 is alsoformed round the top view.

FIG. 4 shows an alternative shape and orientation of the second particlefilter 6 in the housing 14. Here, the second particle filter 6 is formedovally and is arranged inclined to the flow direction of the exhaust gasin the housing 14. Thereby, an enlarged filter-effective surface of thesecond particle filter 6 is provided compared to the verticalarrangement according to FIG. 3.

In an alternative design of the second particle filter 6, which is shownin cross section in FIG. 5, the second particle filter 6 is designed ina cone-shaped manner. A tip 18 of the second particle filter 6 pointsinto a direction against the flow directions of the exhaust gas. The tip18 of the second particle filter 6 lies on a center axis A of thehousing 14, which simultaneously is a center axis of the second particlefilter 6.

In an alternative design of the second particle filter 6, which is shownin FIG. 6 in a top view in the flow direction of the exhaust gas and ina sectional view, the filter membrane 17 of the second particle filter 6facing the exhaust gas flow forms a plurality of concentric elevations19. The support device 16 also has the plurality of concentricelevations 19, so that the second particle filter 6 has a jagged profilein the longitudinal section.

With a further design of the second particle filter 6 shown in FIG. 7,it has a corrugated form in its longitudinal section. A plurality ofparallel elevations 19 of the filter membrane 17 and the support devicesupporting this is formed.

FIG. 8 to FIG. 15 show different arrangements of pressure sensing means,by means of which a difference of the pressure between a locationupstream of the second particle filter 6 and a location downstream ofthe second particle filter 6 can also be sensed as a loading of thefirst particle filter 5. Depending on installation space conditions,access to respective regions of the exhaust gas tract 3 and the type ofthe selected pressure sensors, the pressure drop across the particlefilter 6 can be determined reliably and quickly, in order to conclude adamage of the first particle filter 5.

According to FIG. 8, the pressure sensing means comprise twodifferential pressure sensors 20, 21. The first differential pressuresensor 20 uses a first measurement line 22, which is arranged upstreamof the first particle filter 5. A second measurement line 23 of thedifferential pressure sensor 20 arranged downstream of the firstparticle filter 5 can simultaneously be used by the differentialpressure sensor 21 which senses the pressure drop across the secondparticle filter 6.

With the arrangement of the pressure sensing means shown in FIG. 9, thepressure sensing means 21 senses the pressure downstream of the secondparticle filter 6 is not like the arrangement of FIG. 8 in the region ofthe housing 14, but the second measuring line of the differentialpressure sensor 21 extends into the exhaust gas tract in the region ofthe branch of the exhaust gas return line 8.

With the arrangements of the pressure sensing means shown in FIG. 8 andFIG. 9, an absolute pressure sensor can additionally be provideddownstream of the second particle filter 6. This is particularlysensible if the absolute pressure is to be used downstream of the secondparticle filter 6 as the input variable for a regulation of thelow-pressure exhaust gas return rate through the exhaust gas return line8.

According to FIG. 10, the pressure sensing means comprise three absolutepressure sensors instead of the differential pressure sensors 20, 21.These absolute pressure sensors 24 respectively sense the pressure at alocation upstream of the first particle filter 5, downstream of thefirst particle filter 5, and thus upstream of the second particle filter6 and downstream of the second particle filter 6 respectively within thehousing 14.

With the arrangement of the pressure sensing means shown in FIG. 11,only the third absolute pressure sensor 24 is not arranged in the regionof the housing 14, but at the branch of the exhaust gas return line 8 inthe exhaust gas tract 3.

The arrangement of the pressure sensing means shown in FIG. 12 islargely identical to the arrangement according to FIG. 11, however, anabsolute pressure sensor 24 senses the absolute pressure upstream of thefirst particle filter 5 and the differential pressure sensor 20 thepressure drop across the first particle filter 5 via the measuring lines22, 23. The absolute pressure sensor 24 and the differential pressuresensor 20 use the same measurement line 22.

The arrangement of the pressure sensing means according to FIG. 13 islargely analogous to the arrangement of FIG. 12, but the absolutepressure sensor 24 detects the absolute pressure downstream of the firstparticle filter 5 here and uses the measuring line 23 together with thedifferential pressure sensor 20, which line serves for the measuring ofthe pressure in the housing 14 at the location downstream of the firstparticle filter 5.

With the arrangement of the pressure sensing means according to FIG. 14,the pressure is also present downstream of the second particle filter 6as a direct measurement variable, which is sensed by means of anabsolute pressure sensor 24. However, in the arrangement of FIG. 14, thepressure drop across the second particle filter 6 is determined by meansof the differential pressure sensor 21. This differential pressuresensor 21 uses a measuring line 25 together with the absolute pressuresensor 24, which line extends into the exhaust gas tract downstream ofthe second particle filter 6 at the height of the branch of the exhaustgas return line 8.

With the arrangement of the pressure sensing means shown in FIG. 15, theabsolute pressure downstream of the second particle filter 6 is presentas an indirect measuring variable, which, as with the arrangementaccording to FIG. 14, is sensed at the height of the branch of theexhaust gas return line 8. Hereby, the differential pressure sensor 21using the measuring line 25, by means of which the pressure drop acrossthe second particle filter 6 can be determined, uses a measuring line 26together with the absolute pressure sensor 24, which is used to sensethe absolute pressure between the first particle filter 5 and the secondparticle filter 6.

Provided that an absolute pressure sensor is arranged in the air intaketract 11 upstream of the compressor 12, the absolute pressure downstreamof the second particle filter 6 and the pressure drop across the exhaustgas return line 8 can also be sensed by means of a differential pressuresensor. Its measuring lines are then positioned in the air intake tract11 upstream of the compressor 12 and downstream of the second particlefilter 6. In this case, a measurement of the absolute pressuredownstream of the second particle filter 6 can be omitted with thearrangements depicted in FIG. 12 to FIG. 14. With the arrangement of thepressure sensing means according to FIG. 14, the sensing of thedifferential pressure at the second particle filter 6 by means of thedifferential pressure sensor 21 can however be omitted.

The pressure drop across the second particle filter 6 can becontinuously evaluated by an evaluation device, which can be arranged ina control device (not shown here) for controlling the internalcombustion engine 1. The pressure drop is compared to a comparison valuewhich characterizes a new state of the first particle filter 5. If themeasured pressure drop exceeds the comparison value by a predeterminedthreshold, an alarm is triggered in a monitoring device of the controldevice, which is also called OBD unit (on board diagnosis).

The threshold is presently deposited in the form of a characteristicline, which takes into account the aging of the first particle filter 5.

Additionally or alternatively, for monitoring first particle filter 5,the pressure drop across the second particle filter 6 can be set into aratio to the pressure drop across the first particle filter 5. Themeasured ratio is hereby compared to a ratio characterizing the newstate of the exhaust treatment device 13. The alarm is triggered if thelarge ratio in the new state increases instead of decreasing and/orexceeds the value of the new state. The ratio is preferably formed inconnection to a thermal regeneration of the particle filters 5, 6 takingplace together of the measuring values measured by means of the pressuresensing means.

1-18. (canceled)
 19. An exhaust gas treatment device for an internalcombustion engine, comprising: a first particle filter; and a secondparticle filter arranged downstream of and spaced from the firstparticle filter in a flow direction of exhaust gas through the exhaustgas treatment device, wherein the second particle filter comprises asupport device configured such that the exhaust gas passes through thesupport device; and a filter unit arranged on the support device,wherein a pore size of the filter unit is smaller than a pore size ofthe support device.
 20. The exhaust gas treatment device according toclaim 19, wherein the filter unit is arranged to retain particles bysurface filtration.
 21. The exhaust gas treatment device according toclaim 19, wherein a thickness or a porosity of the one support device isrespectively a multiple of a thickness or a porosity of the filter unit.22. The exhaust gas treatment device according to claim 19, wherein thesupport device is arranged downstream or upstream of the filter unit.23. The exhaust gas treatment device according to claim 19, wherein thesecond particle filter is arranged regeneratably by a heat transfer inan exhaust gas tract within a housing surrounding the first particlefilter, wherein the heat transfer is effected by a thermal regenerationof the first particle filter.
 24. The exhaust gas treatment deviceaccording to claim 19, further comprising: pressure sensing means; andan evaluation device configured to determine a pressure drop across thefirst particle filter and a pressure drop across the second particlefilter.
 25. The exhaust gas treatment device according to claim 20,further comprising: pressure sensing means; and an evaluation deviceconfigured to determine a pressure drop across the first particle filterand a pressure drop across the second particle filter.
 26. The exhaustgas treatment device according to claim 23, further comprising: pressuresensing means; and an evaluation device configured to determine apressure drop across the first particle filter and a pressure dropacross the second particle filter.
 27. The exhaust gas treatment deviceaccording to claim 19, further comprising: an evaluation deviceconfigured to evaluate a difference of the pressure between a locationupstream of the second particle filter and a location downstream of thesecond particle filter with regard to a state of the first particlefilter.
 28. The exhaust gas treatment device according to claim 27,further comprising: a memory storing a reference value, variablydepending on an aging of the first particle filter, of the pressuredifference.
 29. The exhaust gas treatment device according to claim 19,further comprising: pressure sensing means; an evaluation device; and amemory storing a ratio of a difference in pressure between a locationupstream of the second particle filter and a location downstream of thesecond particle filter to a difference of pressure between a locationupstream of the first particle filter and a location downstream of thefirst particle filter, and wherein the evaluation device is configuredto evaluate a ratio determined by the pressure sensing means with regardto a state of the first particle filter.
 30. The exhaust gas treatmentdevice according to claim 24, further comprising: pressure sensingmeans; an evaluation device; and a memory storing a ratio of adifference in pressure between a location upstream of the secondparticle filter and a location downstream of the second particle filterto a difference of pressure between a location upstream of the firstparticle filter and a location downstream of the first particle filter,and wherein the evaluation device is configured to evaluate a ratiodetermined by the pressure sensing means with regard to a state of thefirst particle filter.
 31. The exhaust gas treatment device according toclaim 27, further comprising: pressure sensing means; an evaluationdevice; and a memory storing a ratio of a difference in pressure betweena location upstream of the second particle filter and a locationdownstream of the second particle filter to a difference of pressurebetween a location upstream of the first particle filter and a locationdownstream of the first particle filter, and wherein the evaluationdevice is configured to evaluate a ratio determined by the pressuresensing means with regard to a state of the first particle filter. 32.The exhaust gas treatment device according to claim 29, wherein theevaluation unit is configured to evaluate the ratio determined by thepressure sensing means depending on a thermal regeneration of at leastthe first particle filter.
 33. The exhaust gas treatment deviceaccording to claim 30, wherein the evaluation unit is configured toevaluate the ratio determined by the pressure sensing means depending ona thermal regeneration of at least the first particle filter.
 34. Theexhaust gas treatment device according to claim 31, wherein theevaluation unit is configured to evaluate the ratio determined by thepressure sensing means depending on a thermal regeneration of at leastthe first particle filter.
 35. The exhaust gas treatment deviceaccording to claim 19, further comprising: an exhaust gas return linecomprising a throttle device, which branches off downstream of thesecond particle filter.
 36. A method for operating an exhaust gastreatment device with a first particle filter and a second particlefilter arranged downstream of and spaced from the first particle filterin a flow direction of exhaust gas passing through the exhaust gastreatment device, and comprising a support device configured such thatthe exhaust gas passes through by the support device and on which afilter unit is arranged, the filter unit having a pore size smaller thana pore size of the support device, the method comprising: determining astate of the first particle filter with regard to its functionalitybased on a state of the second particle filter.
 37. The method accordingto claim 36, further comprising: determining a pressure drop across thesecond particle filter; and evaluating, based on the pressure dropacross the second particle filter, whether the first particle filter isdamaged.
 38. The method according to claim 36, wherein during a thermalregeneration with combustion of soot from the first particle filter, thesecond particle filter is also regenerated.